This invention relates to a method for producing tobacco filters. More particularly, it relates a method for producing cigarette filters which exhibit increased cohesiveness between the fibers of the tobacco filter, as a result of heat-induced adhesion.
An acceptable tobacco smoke filter, particularly a cigarette filter, must exhibit a high degree of filtration of tobacco smoke particles, i.e., have high smoke removal efficiency, at an acceptable draw resistance, i.e., pressure drop. The filter must also be capable of economical continuous production. Further it must be at a firmness sufficient to avoid collapse during smoking and must not unduly distort the taste and odor of tobacco smoke. The increasing use of filters in cigarettes not only for the purpose of removing tars and other undesirable substances from the tobacco smoke but also to save the cost of the tobacco which would otherwise be thrown away in the butt-end, has lead to the investigation and development of many kinds of filters. Cigarette filters need to resist damage by high speed making machinery, need to exert less than a certain degree of hindrance to the passage of tobacco smoke on drawing and yet must remove an adequate proportion of the undesirable substances. In addition, the filters should not have such a high pressure drop that the effort to draw smoke through each filter is noticeable to the smoker.
Cigarette filters made of crimped paper or cellulose acetate tow have met with some commercial success, although these entail the use of relatively complex machinery for handling the loose starting materials which must be rolled in paper or otherwise bound together into the desired shape of filter before being incorporated into the cigarette. This particular type of filter can also be comparatively heavy.
Cigarette filters made of cellulose acetate require the use of a costly solvent such as a triacetin solvent, in order to provide desirable adhesion bonding between the fibers of the filter. Such adhesion bonding of the fibers within the filter is important in producing the highly desirable back pressure (the "drag" of the cigarette) which is necessary to effectuate desired filtration of tar and other impurities. During the typical method of manufacture of many filters, air pressure is blown through the fibers to "fluff out" the fibers. The use of 100% polypropylene results in a lack of necessary air resistance, since there is no tackiness or desired adhesion between the fibers. Thus, adequate drag cannot be created.
Also known is the method disclosed in Tamaoki et al, U.S. Pat. No. 4,261,373 which provides a method of making cigarette filters by intially extruding only polypropylene fiber and then extruding separately from polypropylene a second component fiber such as an ethylene vinyl acetate copolymer, forming a fiber bundle of the separately extruded polypropylene fiber and second component fiber, and subjecting the fiber bundle to heat between the melting point of the polypropylene fiber and the melting point of the second component fiber. However, the method in '373 does not direct itself to the unique problem inherent in the use of tobacco filters, in that, the method of '373 suffers from the inherent disadvantage of a low and extremely narrow operating range at which fusion between the extruded polypropylene fiber and the extruded second component fibers can occur in order to get necessarty fiber adhesion.
It is well known that the individual melting points of polypropylene and polybutylene, for example, differ by some 40.degree.-50.degree.l , the melting point of polybutylene being about 130.degree. C. and the melting point of polypropylene being from about 170.degree. C. to about 180.degree. C. depending upon whether the polymer has been stretched. When the lower melting point component (polybutylene, for example) is heated to its melting point which is 40.degree.-50.degree. C. below the melting point of polypropylene, it (polybutylene) begins to melt very rapidly and decrease in viscosity almost immediately, as is typical of crystalline polyolefins. However, while the polybutylene is melting, the temperature is not sufficiently high to reach the fusion point of polypropylene, such fusion point being within approximately 3.degree.-4.degree. C. of the melting point of polypropylene. This results in a melting of the lower melting poing polymer (polybutylene), onto the higher melting point polymer (polypropylene), which is still in a solid form. The only operating range in which the polypropylene can fuse to the polybutylene is the fusion range of approximately 3.degree.-4.degree. C. below the melting point of polypropylene. This results in the necessity of maintaining the operating temperature within an extremely narrow range, as well as constant supervision of the operating temperature control. Thus, operation at a temperature too high results in a fiber puddle much like a puddle produced from melted candle wax, while operation at temperatures too low results in polybutylene melted to solid polypropylene. Such a melt does not result in effective fusion nor does it impart weld strength to the fiber. These disadvantates result from the use of two separatly extruded fibers, each separately extruded fiber continuing to maintain two distinct melting points while in the form of the filter.